Generally, a light flux (brightness) of a lamp represented by a discharge lamp is gradually decreased according to an elapsed time due to a degradation of substances of the lamp by heat and light irradiation, and grime such as dust deposited on the lamp. In the case of a fluorescent lamp, for instance, a fluorescent substance applied to the fluorescent lamp is degraded by generated heat of the lamp during lighting, thereby gradually decreasing a light flux (hereinafter, this phenomenon is referred to as a light flux decline). Therefore, an illumination apparatus configuration and the number of apparatuses to be provided are determined, in general, to obtain brightness more than necessary when the lamp is new, in order to maintain desirable brightness even if a light flux is decreased during a use of the lamp. Thus, an illumination apparatus, in which an accumulated interval of on-time of a lamp is stored and a power supplied to the lamp gradually increases as the accumulated interval of on-time increases, whereby a light flux reduction according to the increase of the accumulated interval of on-time is corrected, has been known. By using such an illumination apparatus, it is possible to achieve desired approximately-constant brightness independent of an operating time of the lamp, and suppress unnecessary brightness when the lamp is new. In addition, it is possible to obtain an advantage of achieving low power consumption by saving electricity to be consumed.
In the illumination apparatus including the so-called “initial illuminance correction function”, it is possible to further save electricity by combination of a light control function that a user can arbitrarily control brightness. With regard to the illumination apparatus with the combination of such an initial illuminance correction function and a light control function, an illumination apparatus as disclosed in Patent Citation 1 has been known, for instance, in which the illumination apparatus is configured to correct a supply power supplied to the discharge lamp obtained by a light flux holding ratio of the discharge lamp according to an accumulated interval of on-time, with a light control ratio externally specified, so as to determine the supply power to the discharge lamp. Specifically, when the supply power obtained by the light flux holding ratio according to the accumulated interval of on-time is 70%, and when a light control signal to specify 50% of a light control is externally input, the illumination apparatus is configured to turn on the discharge lamp with a power of 70%×50%=35%. Due to such a configuration, it is possible to relatively easily obtain the illumination apparatus configured with the combination of the initial illuminance correction function and the light control function.
A specific operation example of the illumination apparatus with the combination of the initial illuminance correction function and the light control function will be described with reference to FIG. 1 and FIG. 2. As shown in FIG. 1(a), a maximum supply power to a lamp, which can be output by the light control function, is limited to approximately 70% of a rated power of the lamp when the lamp is in an initial lighting point (the accumulated interval of on-time is 0 hour). Note that, the maximum supply power at the initial lighting point is arbitrarily set so as to suppress unnecessary brightness at the initial lighting point based on a light flux decline property of the lamp. In addition, as shown in FIG. 1(b), when a minimum light control ratio of the light control function is 25%, for instance, a minimum supply power to the lamp, which can be output by the light control function, is limited to 70%×25%=17.5% when the lamp is in the initial lighting point (the accumulated interval of on-time is 0 hour).
When the supply power gradually increases so as to correct lowered brightness due to the light flux decline of the lamp as the accumulated interval of on-time of the lamp increases, and when the accumulated interval of on-time reaches a rated lamp life (12,000 hours in this case) or near, the maximum supply power shown in FIG. 1(a) increases to 100% (the rated power of the lamp or near). In addition, the minimum supply power shown in FIG. 1(b) increases to 100%×25%=25%, in proportion to the increase of the maximum supply power.
Next, FIG. 2 shows a relationship between the light control signal and the amount of the supply power supplied to the lamp. As shown in FIG. 2(a), when the lamp is in the initial lighting point (the accumulated interval of on-time is 0 hour), the supply power to the lamp is 70% that is the maximum supply power in the initial lighting point when the light control signal has the maximum value (FULL). In addition, the supply power to the lamp is 17.5% that is the minimum supply power in the initial lighting point when the light control signal has the minimum value (DIM). Moreover, as shown in FIG. 2(b), after the accumulated interval of on-time reaches the rated lamp life (12,000 hours in this case) or near, the supply power to the lamp is 100% that is the maximum supply power when the light control signal has the maximum value (FULL), and the supply power to the lamp is 25% when the light control signal has the minimum value (DIM).
Due to such a control process as described above, the illumination apparatus concurrently having the initial illuminance correction function and the light control function is provided.    Patent Citation 1: Japanese Patent Application Laid-Open Publication No. 2001-015276